Many industrial processes utilize steam created by a boiler that is fed by a solid fuel such as coal, wood, biomass, or other similar material. Such fuels, when combusted, produce ash and other fine particulate matter as by-products which must be removed from the flue gas of the boiler prior to release of the flue gas to the atmosphere. Acid gas emissions may also be present. A wet electrostatic precipitator (WESP) is often used to remove particulate matter from the flue gas, in the presence or absence of acid gas emissions.
As described, for example in U.S. Pat. Nos. 7,297,182 and 7,318,857, commonly owned with the present application, a wet electrostatic precipitator requires a supply of water for quenching the flue gas. Most of this water is evaporated into the flue gas and thus exits the WESP into the atmosphere, but a portion of this water is discharged from the WESP as bleed water. The bleed water has historically been handled in several different ways, including disposal through a municipal sewer system, disposal through a water treatment facility, disposal to a settling pond, and processing in commercially available equipment that includes centrifuges and evaporators. Disadvantages of these prior methods for disposal of the bleed water include, but are not limited to, problems with environmental permit compliance (especially for zero liquid discharge facilities) and high cost of operation for centrifuge and evaporator systems.
A steady supply of fresh makeup water is typically required to replace the water evaporated into the flue gas and water discharged as bleed water, and a steady stream of waste effluent comprising bleed water must typically be treated and/or disposed of. For a system of industrial scale, the cost of supplying the fresh makeup water and the cost of treating and/or disposing of the waste effluent can be substantial.
Further, a steam boiler is typically periodically subjected to a blow down operation in which an amount of water in the bottom of the boiler is discharged in order to reduce the concentration of contaminants such as solids and chloride that could have detrimental effects on the operation of the boiler and related equipment. The blow down water is waste effluent that typically must be treated and/or disposed of, again at a substantial cost due to the sheer quantity of waste effluent that is generated for a boiler of industrial scale.